We saw last time one of the deadlocks you can run into when playing with pipes. Today we’ll look at another one:
Our program runs a helper process with stdin/stdout/stderr redirected. The helper process takes input via stdin and prints the result to stdout. Sometimes we find that the
WriteFilefrom the controlling process into the stdin pipe hangs. Closer examination reveals that the helper process no longer exists. Under these conditions, should the
WriteFilefail, since the reader is no longer available?
If you attempt to write to a pipe when
there is nobody around to
ReadFile to read the data out the other end,
the call to
WriteFile should fail with the error
(known in Unix-land as
What does it mean when the write pends?
It means that there is still somebody around who can read the data
out of the pipe,
but the internal pipe buffer is full,
so the write call waits for the reader to drain the data.
But the helper process no longer exists. Maybe it crashed or exited prematurely. That means that there is nobody around to read the data out of the pipe. Why, then, does the call not return immediately with an error?
Because there is still somebody around to read the data out of the pipe.
Did you remember to close the controlling process’s copy of the read end of the pipe?
If the controlling process hasn’t closed its copy of the read end
of the pipe, then the pipe is correct in believing that there is
still somebody around to read the data out of the pipe,
You have a handle to the read end of the pipe,
so the pipe manager cannot declare the pipe dead;
for all it knows,
you intended for the controlling process to call
ReadFile to read the data out of the pipe.
As far as the pipe is concerned,
you simply haven’t gotten around to it yet,
so the pipe waits patiently.
Yes, our code calls
CloseHandleon the controlling process’s copy of the pipe handles. I’ve highlighted it below. (Error checking has been elided for simplicity.)// create the pipe for stdout/stderr CreatePipe(&hReadPipeTmp, &hWritePipeTmp, NULL, 0); // duplicate the handles with bInheritHandle=FALSE to prevent // them from being inherited DuplicateHandle(GetCurrentProcess(), hWritePipeTmp, GetCurrentProcess(), &hWritePipe, 0, FALSE, DUPLICATE_SAME_ACCESS); DuplicateHandle(GetCurrentProcess(), hReadPipeTmp, GetCurrentProcess(), &hReadPipe, 0, FALSE, DUPLICATE_SAME_ACCESS); // create the pipe for stdin CreatePipe(&hHelperReadPipe, &hHelperWritePipe, NULL, 0); // disable inheritance on on the write end of the stdin pipe SetHandleInformation(hHelperWritePipe, HANDLE_FLAG_INHERIT, 0); // prepare to create the process ... blah blah blah other stuff unrelated to handles ... startupInfo.hStdInput = hHelperReadPipe; startupInfo.hStdOutput = hWritePipeTmp; startupInfo.hStdError = hWritePipeTmp; CreateProcess(...); // Here is where we close the handles CloseHandle(hReadPipeTmp); CloseHandle(hWritePipeTmp); // Write the input to the helper process (hangs here sometimes) WriteFile(hHelperWritePipe, ...);
This is another case of getting so excited about doing something that you forget to do it. (Notice how the comments to that article very quickly descend into a discussion of command line quotation marks.)
Observe that the handles being closed are
hWritePipeTmp, which is a good thing to do,
but neither has any effect on the
WriteFile is writing to
and therefore the handle you need to close is
Since that handle is still open in the controlling process,
the pipe manager will not break the pipe,
because it’s waiting for you to read from it.